1. Field of the Invention
This invention relates to the corrosion protection of the internal surfaces of pipes, in particular to an arrangement for providing such protection.
2. Summary of the Prior Art
Protection against the corrosion of metals, more particularly iron and steel, has been provided successfully and at low cost by cathodic protection either by the use of sacrificial anodes or by using permanent anodes coupled to a potential source which is used to overcome and reverse the natural potential difference which is set up whenever a metal is immersed in an electrolyte such as sea water. Apparatus and systems have been evolved which satisfactorily protect ships, liquid-containing tanks, bridges and other structures and more recently the external surface of ducts and pipe lines has also been successfully protected.
However, wherever sea water is passed through pipe lines the problem of internal protection is acute and to give reasonable installed life under these circumstances it has been necessary to make the pipes substantially thicker than strength considerations would otherwise call for in order to avoid frequent and costly replacement. It has been found that 12 inches (30.48 cm.) diameter pipes of spun cast iron may corrode to a degree that they become unserviceable within one year or even substantially less time under certain conditions. Since the pipes are costly in themselves and the labour costs of replacing the pipes and in certain cases the apportioned costs of taking a ship out of commission are at least as high, the need for providing effective and inexpensive internal protection is fully apparent. The ever-increasing use of very large size oil tankers has emphasized this problem since such tankers have extensive pipe work.
The main pipes of such tankers can be divided into three categories. One set of pipes (cargo lines) is uses to communicate with the tanks for the purpose of taking on and discharging the main bulk of the cargo from these tanks and subsequently ballasting operations within these tanks. Another pipe, (ballast line) is primarily used for loading and unloading sea water ballast from the permanent ballast tanks. A third set of pipes (stripping lines) is usually used for removing residual oil or ballast. The stripping lines are more likely to be subject to corrosion since the residue left in the tanks often has a high proportion of sea water and, of course, the ballast lines also suffer from the same problem. On some vessels the cargo/stripping lines are combined and structurally are formed by a duct which is defined by structure of the vessel. Such ducts also require protection.
Problems similar to those encountered in sea-going tankers and other vessels also occur in offshore rigs and pipelines where sea water is sometimes static within pipes for long periods. Recent experience has shown that the rate of corrosion can be very high under certain circumstances, and replacement costs can be far higher than for a sea-going vessel which can be conveniently docked. In all pipe lines roughness causes reduction in capacity and it is therefore essential that flow losses should be kept to a minimum since otherwise the size of pipe for a given flow rate needs to be increased.
It is an object of the present invention to provide an anode arrangement for a cathodic protection system which minimises the flow disturbance within the pipe.